After giving it some thought, how about the formula below as perhaps a better ROT?
It is reasonably conservative and hopefully a bit easier to remember .
Lpt <= 1000 / fr
Where:
Lpt is the port tube length in centimetres
<= means less than or equal to
fr is the bass reflex system Helmholtz resonance frequency in hertz
Assuming that the speed of sound is 344 metres per second:
If fr = 40 Hz then maximum Lpt = 25 cm
If fr = 50 Hz then maximum Lpt = 20 cm
Just to clarify, the "Assuming that the speed of sound is 344 metres per second:" statement should have been positioned just above the Lpt <= 1000 / fr formula.
So, a BR tuned to 40 hertz and has a port longer than 10 inches (25.4cm) equals a TL?
If the speed of sound is 344 metres per second then a conventional transmission line loudspeaker would require the line to be at least 215 cm long, or 1/4 wavelength (WL) at 40 hertz.
The rule of thumb formula can be generalised to:
L = A / fs
Where:
L = duct length in cm
A = ROT factor
fs = system tuning frequency in hertz
If the speed of sound is 344 metres per second then:
If A <= 1000 (L <= 0.03 * WL) the loudspeaker will be a bass reflex (BR) system
If A >= 8600 (L >= 0.25 * WL) the loudspeaker will be a transmission line (TL) system
If A > 1000 and A < 8600 the loudspeaker will (perhaps) be a birtle (BRTL) system...
Could a slot loaded open baffle be modeled?
I have never tried, but I can't see why it should not be possible to come up with a reasonable model.
Someone is bound to have attempted it - Google is your friend...
If the speed of sound is 344 metres per second then a conventional transmission line loudspeaker would require the line to be at least 215 cm long, or 1/4 wavelength (WL) at 40 hertz.
The rule of thumb formula can be generalised to:
L = A / fs
Where:
L = duct length in cm
A = ROT factor
fs = system tuning frequency in hertz
If the speed of sound is 344 metres per second then:
If A <= 1000 (L <= 0.03 * WL) the loudspeaker will be a bass reflex (BR) system
If A >= 8600 (L >= 0.25 * WL) the loudspeaker will be a transmission line (TL) system
If A > 1000 and A < 8600 the loudspeaker will (perhaps) be a birtle (BRTL) system...
I was thinking 7ft was the magic number due to Pro Audio FLH's are usually around that length.
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I would be interested to learn more about the mathematical model behind HornResp, and even the history behind how it was first implemented. I read on the internet somewhere "The Hornresp program, written by David McBean and based on Olson's horn model, is a very easy to use horn simulation program. David wrote the original version in the early 1970's in Fortran IV and ran it on a room-sized IBM mainframe computer." which quite fascinates me. But I am not sure which paper by Olson I should read and I've no idea how a solution would be implemented in Fortran (I last used Fortran IV as a teenager and have a interest in old IBM mainframe technology). I understand some or much of this maybe deemed proprietary, but if it's not a secret I'd appreciate some pointers on where to find this information (I've searched this thread for 'formula' but that didn't give me much satisfaction).
Maybe when you start using feet or meters instead of inches or centimeters???
1964 with 1984 publishing.
No BR port is longer than 12".
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